1. Field of the Invention
The present invention is directed to a method and apparatus for determining whether electrical signals in a heart are caused by an atrial depolarization or a ventricular depolarization of the type wherein electrical signals are sensed with an electrode surface in contact with ventricular heart tissue and the sensed electrical signals are analyzed to distinguish whether they were caused by atrial depolarizations or were caused by ventricular depolarizations.
2. Description of the Prior Art
In a healthy heart, a heartbeat commences with a spontaneously generated electrical impulse in the sinus node. The impulse first induces atrial depolarization resulting in contraction of atrial heart muscle (atrial systole). The contraction of atrial musculature pumps blood from the respective atrium down to the ventricles. After a delay, the impulse is then conducted to the ventricles via the A-V node and causes ventricular depolarization, resulting in a contraction of ventricular heart muscle (ventricular systole). The contraction of ventricular muscle expels blood out of the heart into the systemic circulation and the pulmonary circulation. Heart tissue in the atrium and ventricle then repolarizes, and the muscle tissue relaxes (diastole). The heart then refills with blood from the veins. As the sinus node generates a new impulse, the cycle restarts.
For people with certain heart defects, a pacemaker can be an excellent aid which is capable of taking over stimulation of heart tissue when natural, spontaneous self-stimulation is inoperative. When e.g., the A-V node blocks transmission of signals from the atria but the sinus node works properly so as to trigger atrial depolarization in a natural way, a pacemaker capable of stimulating ventricular heart tissue, so the stimulated heart cycle resembles a healthy heart's cardiac cycle as closely as possible, is advantageous.
In order to achieve such pacing, it must be possible to detect atrial depolarizations at the same time as it must be possible to deliver stimulation pulses to the ventricular tissue. For this purpose, the use of dual chamber pacemakers, in which a first electrode with an electrode surface is placed in the right atrium to sense atrial depolarizations and a second electrode with an electrode surface is placed in the apex of the right ventricle to stimulate the ventricular tissue, is known. One such pacemaker is described in U.S. Pat. No. 4,343,311. At least two electrodes must be introduced into the heart, making the system more complex. Complications can develop during the implantation of the electrodes, and there is an increased risk of faults. In addition, a plurality of electrodes impedes the return of blood to the heart to some extent.
In order to reduce the number of electrodes which must be introduced into the heart via the venous system, a multipolar electrode can be employed having at least one electrode surface in the ventricle to stimulate same and at least one electrode surface in the atrium to sense atrial depolarizations, one such electrode is described in U.S. Pat. No. 4,444,195. For reliable detection of atrial depolarizations, the electrode surface in the atrium should be in close proximity to atrial tissue. The electrode must therefore be so stiff that it remains in virtually constant contact with atrial tissue. This could irritate heart tissue during the muscular contractions. Also, in general, a multipolar electrode also has a larger diameter than a unipolar electrode.
Both when a plurality of electrodes, each with one electrode conductor, is used and when multipolar electrodes, with a plurality of electrode conductors, is used, conductor breakage is always a risk. The more conductors employed, the greater the likelihood that one of them has a slight defect or reduced abrasion resistance, despite careful fabrication and quality control.
In U.S. Pat. No. 4,905,696 a detector is described which unipolarly measures heart impedance in the ventricle, and from the impedance signal atrial depolarizations can be identified as fast, brief changes.
In a Swedish patent application No 9203642-5, corresponding co-pending U.S. application Ser. No. 08/152,126 filed Nov. 16, 1993 entitled "Detector for Sensing Events in Living Tissue" (Sivard et al.) and assigned to the assignee of the subject matter claimed herein, a detector is described which has at least two integrators. The heart's electrical signals are sensed. Monophasic and biphasic signals can be distinguished by the respective integrators integrating signals with different polarity. Since depolarizations in the atrium are monophasic when the signal is sensed in the ventricle and ventricular depolarizations are biphasic when sensed in the ventricle, this detector can distinguish between depolarization in the atrium and ventricle respectively on the basis of electrical signals detected unipolarly in the ventricle. The aforementioned patent application, however, like the other prior art described, does not consider the way the signals should be picked-up so that the most reliable discrimination which is possible is achieved. This co-pending application, nor the prior art, moreover do not address the problem of how to reduce the irritation to which heart tissue is subjected due to employed electrodes.